A foreign object detector is known through Japanese Kokai Patent Application No. 2003-240525, by example. In the case of this foreign object detector, light is irradiated onto a road surface, catoptric light is received, and image processing is applied to the catoptric result to obtain a moiré image. Then, points on moiré fringes in the moiré image are triangulated to obtain their distances to separate the coverage area where the object is present from the coverage area and where the background is present within the image in order to detect a concave or convex object.